The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
Hydraulic motor vehicle transmissions, that is, automatic transmissions for passenger cars and light duty trucks having a plurality of planetary gear assemblies controlled by clutches and brakes, generally include a dedicated hydraulic pump which provides pressurized transmission (hydraulic) fluid to control valves and actuators which engage the clutches and brakes and provide the various gear ratios or speeds.
Such dedicated pumps are generally fixed displacement pumps such as vane or gear pumps that are driven at engine speed from the hub of the torque converter or other startup device located between the engine and the transmission. Such pumps have many design goals. Since the pump is constantly driven at engine speed, it is desirable that it have high efficiency. Additionally, since the pump is most frequently mounted concentric to the engine axis, small size, particularly axial length, is desirable in order not to increase the length of the transmission. Such an on-axis engine driven pump must also be self-priming and must function reasonably well under cold start conditions when the transmission fluid has high viscosity because until hydraulic pressure is established, the transmission may be unable to shift into any gear.